Construction of gasometer roof tanks



Nov. 17, 1936. E. H. STABER p 2,061,175

CONSTRUCTION OF GASOMETER ROOF TANKS Filed Jan. 26, 1934 2 Sheets-Sheet l ATTORNEY NW2, 1?, 1936. 5 H STABER 2,061,175

CONSTRUCTION OF GASOMETER ROOF TANKS Filed Jan. 26, 1954 2 Sheets-Sheet 2 r L 23 26:, 1: .1 97 I r if ATTO R N EY Patented Nov. 17, 1936 CONSTRUCTION or easomz'rnn noon 'rmvxs I Ernest H. Staber, Manhasset, N. Y., asslgnor to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York Application January 26, 1934, Serial No. 708,351

.2 Claims.

This invention is directed to improvements in storage tanks for volatile liquids, and more particularly, is directed to improvements in tankshaving a gasometer type roof.

The gasometer roof tank is a tankin which the roof has a depending skirt which dips into a trough extending around the circumference of the tank wall proper, either inside or outside, at the top. This trough or well is filled with liquid, and the tank roof is free to rise and sink in accordance with the vapor pressure below. The application of excess pressure and vacuum is avoided by the use of proper relief valves.

One of the difiiculties experienced with gasomet'er roof tanks has been the escape of condensed' liquid through the circumferential well, which has taken place as follows: The metallic roof, in contact with cool air on the outside (at night, etc and with hydrocarbon vapors on thednside, causes these vapors to condense on the under side of the roof.' Due to the pitch of the roof, they run to the edge, drip into the well and eventually fill it on the inside.

Other difliculties had withthese roofs have been the provision of proper gauging means whereby the measurement of the contents of the tank might be eifected, without allowing the escape of vapors or without attempting to find a time when the tank was not. .under pressure or vacuum and gauging at that time.

In addition, pressure and vacuum -relief valves of the types commonly supplied have not been properly applicable to .the problems presented by gas'ometer roof tanks.

The object of this invention has been to devise a gasometer roof tank in which the loss of stored fluid by escape through the dip ring seal has been prevented. A further object has been the provision of reliable and easily operated gauging means, and the provision of -reliable pressure and vacuum relief valves. Ad ditionally, other objects and advantages have been attained, as may hereinafter appear.

The various objects and advantages are obtained by means of the designs set forth in this specification and shown in the drawings attached to and made a part hereof. 1

In these drawings,

Figure 1 is a section through the dip ring seal, Figure 2 is a sectionof a portion of the dip rin Figure 3 is a diagram showing the action of the dip ring seal when venting,

. Figure '4 is a diagram showing conditions when dip ring is at its low point,

Figure 5 is a diagrammatic section showing a gauge well, I

Figure 6 is a diagram showing relief valves, 5 Figure 7 is a section through one relief valve chamber.

Referring now to the manner in which the 1081 of condensate collected in the dip ring seal is avoided, the method of operation is based on 10 the following:

Being lighter than and not soluble in the liquid within the trough, the condensate will col- -"lect and float on the surface of said liquid and sides of the dip ring will always be maintained as shown by the following formulae Hydrostatic liquid pressure on outside of dip ring=hydrostatic pressure of sealing liquid and superimposed gasoline on inside of dip ring. 20 plus the existingvapor pressure within the tank.

As the gasoline condensate accumulates, the plane where the sealing liquid and gasoline meet lowers, and the upper surface of the gasoline 25 rises.

Y The pressure required to lift the gasometer tank roof determines the venting position of the lower edge of the dip ring. For example, if water is used as the sealing liquid and a pressure 30 equivalent to that of a column of water three .inches in height is required to raise the roof,

the tank will vent by vapor bubbling through the water underneaththe dip ring when the lower edge of said dip ring has risen to within three 5 inches of the surface of the water on the outside of .this dip ring. If gasoline has accumulated on the inside of this dip ring below this point, the instant the lower edge of the dip ring in rising, passes the plane where water and gasoline 40 meet, gasoline will flow underneath the dip ring to the outside surface of the sealing liquid where it readily evaporates. In addition to losses incurred through the evaporation of this releasedcondensate, fire risks and venting are materially increased.

To stop this gasoline from' escaping in this manner is one of the major obiects of this invention, the several objects of the several pertinent portions of the. invention being prevention of escape of gasoline to the outside portion of the well, minimizing of drip accumulating in the inner portion of the well, and retumingsuchdripasdoeaaccumulatetothe' or box.

body of the tank, and such other objects and advantages as may hereinafter be shown.

These objects have been accomplished large- 1y. by taking advantage of the relative specific gravities of water and of gasoline. In order to better understand the manner in which this has been done and the means by which it is accomplished, reference is made to Figures 1 to 4 inelusive.

In Figure 1, I is the tank wall bounded circumferentially inside at the top by stiffener member l4. Attached to the exterior of the tank shell is a circumferential well composed of horizontal plate 3 and vertical plate 4, which in turn has a girder member 5. At various points around the circumference of the tank on the inside of plate 4, there are installed sealing liquid overflow boxes 6, from which said liquid is led away by pipes 1. Depending into the circumferential well is the dip ring sheet 8 attached to the roof 9. At intervals on the .interior of the dip ring sheet are placed vent boxes; in. I have made these vent boxes of a trough or channel-shaped section of iron turned so that its legs are in contact with dip ring 8, and have welded it to the dip ring, forming a vertical box of for the passage of gases. tom of this box by a welded plate I I, but this closing is not essential, and have formed hole l2 in plate 8 so that gases escaping may pass down through the interior of member i and through hole l2. On the interior of roof .9, at a point near the outer edge thereof, I have placed a light angle iron 13, the purpose of this; angle iron being to form a drip point for the condensate collected on the interior of the tank roof inside of the circle formed by l3. In this manner I am enabled to keep the major portion of the condensate from getting into the circumferential well, only that condensate formed between 1 3 and 8 being able to drop into the well. Figure 2 is a section of a portion of 'dip ring 8 showing channel Ill and hole l2.

The objects of this vent box are as follows:

1; Stop vapor escaping by bubbling underneath dip ring.

2. Force vapor to escape by passing through vent box I 0, and hole i2 through which condensate cannot escape. This venting will take place when thereof has risen to a point where the liquid pressure above said hole is equal to the pressure required to raise the roof.

3. Use the bottom of dip ringas trap to prevent the escape of condensate by limiting the height to which dip ring.can rise. This is accomplished by locating vent hole I 2, "a desired distance above bottom edge of dip ring as determined hereinafter mathematically.

4. Automatically force gasoline condensate which collects on the surface of the sealing liquid on the inside of dip ring, back into the tank by flowing over the top of the tank or through a properly located overflow hole, notch This is accomplished by taking advantage of the diiference in specific gravity of the condensate and the sealing liquid.

To illustrate, the following assumptions are made, from which the vertical location of hole I2 is determined algebraically:

Let a=vertical distance in inches between top of tank at It and top of o'verflowbox 6.

Let p=pressure in inches of water required raise roof.

Let s=specific gravity of condensate,

maximum, 22". J between minimum and maximum pressures is of Let h=maximum depth in inches to which condensate can accumulate on inner side of dip ring.

Let w=distance in inches between bottom edge of dip ring and top of overflow box 6 (sealing liquid surface) when roof is in venting position.

w also=h-a,- Therefore If the condensate is gasoline which has a spev cific gravity of 0.75, if 10:3 of water, which is approximately what is found in practice and a=2 then and h and b can also be determined for-any pressures below that required to lift the roof (which is the maximum) but for all such pressures both h and b will have lower values making such calculations unnecessary. Y

- Experience has shown that the time which elapses between maximum and minimum vapor pressures within the tank are of short duration, during which period very little condensate can collect in the water seal, particularly if roof is equipped with drip angle. As a drop in pressure (9) reduces the depth of accumulated condensate, any condensate which accumulates while the roof is in a raised position gradually reduces as the pressure drops and reaches a minimum when the vacuum valve opens to admit air.

These vacuum valves are usually designed toopen when pressure within the tank is 1 of water below that of the outside atmosphere. If this value of p is used, in place of 3" in the above example, then Thus the minimum value for h is 4" and the Since the period which elapses short duration, during which time very little condensate can accumulate, in practice it is found unnecessary to sacrifice 16 of the lift by placing the vent hole 16 from bottom edge of dip ring. It has been found that it is practical to reduce this distance to 6".

In'order to prevent sealing liquid from flowing over the top of the tank into the gasoline, the distance a is made considerably greater than the vacuum in inches of water at which the.

gauging by means of the device set forth diagrammatically in Figure 5, wherein, as before,

' I is the shell of the tank, 3 and 4 form the liquid seal around the upper edge of the shell, 8 is the dip ring at .the edge of the roof, and 9 is the roof. At some point within the tank I have erected a vertical pipe l5, terminating at or near the bottom of the tank and closed on the bottom by a plate 16. In small tanks, or in tanks arranged so that the roof rotates as it rises, this pipe should be at the center. In other tanks, a location near the side wall is preferred. In the lower portion of pipe l5, for a distance of say 18 inches or two feet, I have perforated the. pipe-with a plurality of small holes l1, the arrangement and total area of these holes being such that the liquid within the pipe is always in free communication with the liquid outside of the pipe and the total area being sufliciently small so that momentary surges in the exterior liquid, due to pumping, sudden pressure changes, and the like, are not communicated to the interior of the pipe in the form of surges. Around the top end of this pipe 15 at a level near that of the seal on the outside periphery of the tank wall, I have provided an interior seal composed of a pipe i8, considerably larger in diameter than pipe I 5, which'is placed around pipe 15 and secured thereto by plate. l9, which forms the bottom of an annular liquid retaining space concentric with and exterior. to pipe i5. I connect this liquid space to the liquid in the exterior seal by pipe 20, through which liquid may flow freely. Inside of the seal formed by pipe l8, and exterior to the pipe I5, is a pipe 2i depending from and supported by the roof 9. I have thus set up at this point a trap for vapor of the. same nature as that on the tank wall, with the following exception. The vertical length of pipe 18 is greater than the vertical 'depthof the exterior trap, and

the pipe 2| extends into the'liquid in seal- 18 to a greater depth than does the dip ring 8 in the exterior seal; consequently, any venting of vapors will occur at the exterior trap rather than in the gauge well. The exterior portion of the pipe 2| is closed by a hatch 22 of any convenient form, preferably with a protecting lid. This lid should not be vapor tight, but should be vented to the atmosphere to protect against vacuum and pressure. In gauging the tank, a pole or tape 23 is inserted through this gauge hatch 22 and into the interior pipe I 5 until it is stopped by touching bottom, after which it is withdrawn and the depth to which it is wetted is noted, this giving the height of the level 24. This-level 24 is not the same as the actual level 25 of the liquid within the tank, the difference being that amount 26 due to the variation between internal and external pressure, an amount which may be either plus or minus. To observe the amount of this difference 26, a quantity of the 4 liquid from within the tank is withdrawn through the gauge hatch and placed in the' manometer 21 which is arranged upon the root at a point convenient to the gauge hatch. In this manometer the diflerence 26 is reproduced, and as the liquid in the manometer is of exactly the same nature as that in the tank, no corrections for specific gravity and the like need be made. The difference 26 observed in the manometer is applied to the level 24 observedby means of the gauge pole and the correct level 25 thus determined, and by reference to a computed table of tank capacity, the gallonage at this level is determined.

'To protect this tank against vacuum and against pressures which might not be properly relieved due to failure of operation of the roof, or to other causes, I make use of protective vacuum and pressure relief valves which are set forth diagrammatically in Figure 6. In this figure, 28 and 29 are valve chambers cylindrical in cross section, set up adjacent the top of the tank wall in a convenient operating location, and at a height such that they may communicate with within chamber 28 to a level somewhat greater than the level of the water overflow on the dip ring trap. Over this central pipe 33 and surrounding it, there is placed a gasometer float 34 which extends in its normal position into the liquid in the vessel 28 to a depth of say 22 to 24 inches. .This gasometer float 34 is made of a light weight metal such as aluminum and its weight is adjusted by machining so that it will In other.

rise. and vent at the proper pressure. words, if the dip ring vent is designed to operate at say 3 inches of water and it is decided that the safety vent shall operate at five inches of water, the weight of the gasometer float 34 is such that a presure of five inches of water exermd upon its area will lift it. allowing gas to escape under the bottom of its skirt. In the design of valves for use on tanks working at high pressure, heavier metals, such as steel might be used. 'I'he skirt of this gasometer float 34 is provided with a plurality of ribs 35 which serve to guide it within chamber 28. Chamber 28 is open to the atmosphere through an orifice 31 at its top, and is of sufllcient height to handle the full rise of gasometer float 34 and still retain it in position to properly return to its seat when through venting. Chamber 28 is connected to the pressure space within the tank by inlet 32 previously mentioned, and by pipe 38 which ter- 'minates within the tank at apoint well up in the vapor space near the roof. For the relief of vacuum, a similar chamber 29, equipped with a similar'interior pipe 39 and a similar gasometer float 40, is used, with the difierence that in this case the port 4| at the bottom of chamber 29 is open to the atmosphere, and the top of the chamber 29 is, closed to the-atmosphere and in communication with vent line 38. The gasometer float 40 has similar ribs 36 and in this case is machined so that its weight enables it to act at the desired vacuum, which may for example be 1 of water, so that when a vacuum greater than this amount exists within the tank, the

gasometer float 40 will rise and air admitted through 4i will pass upward through pipe 39, the

gasometer float will rise to operating position,

and the air will then pass downward around sets forth in greater detail the location of the seal to handle a five inch pressure variation; and i ribs 36 upon the side of the gasometer fioat 34 and the annular relationship of orifice pipe 33 and gasometer float 34 Certain advantages of this form of relief valve will at once be apparent. In the past, relief valves of this type have been made so that they vented with a comparatively short rise and the rapidity with which they acted resulted in a considerable blowing of the sealing liquid. Many liquids have been used as seals ranging from mercury through glycerine to 1 water and light petroleum fractions. None of them have been eminently, successful, yet a liquid seal valve is so much more desirable than one sealed by mechanical seating of the valve, that the type has persisted in spite of its defects. These disadvantages I have avoided in several ways: First, I have eliminated the most frequently performed function, namely, that of pressure venting for ordinary rises, and do not call upon the vent valveto perform this routine venting, and reserve for it only the duty of emergency relief, which can only occur when excessive binding takes place between roof guides and rolls, (which condition should be immediately corrected). Secondly, I have provided a vent valve on the gasometer principle, having a depth comparatively great in relation to the necessary movement of the valve. For instance, in the pressure valve of the example cited, I have a 22 inch depth of in communication with and in a position ,to be" replenished by the enormously larger reservoir eating with the liquid space on the outside of said dip ring by an aperture whereby, when venting, the tank gases preferentially may pass into the upper end of thevent box, down through said box, and out through said aperture to a region of lower pressure. -2. In a gasometer roof storage tank for volatile liquids wherein a dip skirt depends from the roof into a circumferential liquid seal well, a liquid in such seal which is not miscible with condensate from'the vapors within the tank and of greater specific gravity than such condensate, a plurality of apertures in said dip skirt adjacent the bottom edge, thereof, passages communicating between said apertures and the vapor space within the tank, an overflow to limit the rise of seal liquid in the space external to the dip skirt, said overflow being not less than two and onehalf inches below the top of the tank wall which defines the inner side of the seal chamber, the apertures being not more than sixteen inches nor less than four inches distant from the bottom edge of the dip skirt, and the weight of the roof being not less than about five pounds per square foot of proiected area exposed: to the pressure of the vapors within the tank.

- ERNEST H. STABER. 

